Devices of the abovenamed type can be used, inter alia, in filling level metrology. There, microwaves, are transmitted by means of an antenna to the surface of a filled material, and the echowaves reflected at the surface are received. An echo function representing the echo amplitudes as a function of distance is formed, and is used to determine the probable useful echo and the propagation time of the latter. The spacing between the surface of the filled material and the antenna is determined from the propagation time.
It is possible to use all known methods which permit relatively short distances to be measured by means of reflected microwaves. The best known examples are pulsed radar and frequency-modulated continuous wave radar (FMCW radar).
It is normal to use horn or rod antennas in filling level metrology. Horn antennas have a waveguide on which a funnel-shaped metal horn is integrally formed in the direction facing the filled material. Rod antennas likewise have a waveguide. However, in this type of antenna there is inserted into the waveguide a rod which is made from a dielectric and extends in the direction facing the filled material. Both types of antenna are usually fed via a coaxial line, which is connected to an excitation element projecting into the waveguide. For the purpose of the electrical connection of the excitation element, use is made of relatively expensive, commercially available plugs and sockets, e.g. of type SMA or N. Such connecting elements are not only expensive, but also cause power losses, since there is present at each transition an impedance jump at which a proportion of the microwaves is reflected. This reflected proportion is no longer available for measurement as useful microwave energy.
The excitation element is, for example, a transmitting pin which is inserted laterally into a circular waveguide and through which electric field components are excited. Here, laterally means perpendicular to the longitudinal axis of the waveguide.
Recent developments by the applicant have shown that it is possible to use a transmitting wire as excitation element. Reference is made in this regard to the German patent application filed on Jul. 23, 1996 and having the file number 196 29 593. The transmitting wire described there has a straight section and two limbs adjoining thereto. The transmitting wire is inserted into a circular waveguide in an axial direction. Magnetic field components are excited by the transmitting wire.